1. Field of the Invention
The present invention relates generally to scratch protection for integrated sensor circuits, and more specifically to improvement of scratch protection in capacitive sensor circuits through selection of metallization materials.
2. Description of the Prior Art
Fingerprint acquisition circuits employ arrays of sensors coated with a dielectric onto which the finger is placed with the epidermis in contact with the dielectric. The ridges and grooves on the epidermal layer of the finger are then detected by the sensors, which transmit signals representative of the detected pattern. Although various sensors are possible (e.g., resistive, etc.), capacitive sensors have been found to provide the best performance. Since capacitance between two capacitive plates is inversely proportional to the distance between the plates, using the contacting dermal tissue itself as one capacitor plate and the sensor electrode as the other and then determining capacitance for each sensor electrode in the array, it is possible to locate the ridges and grooves of the fingerprint.
Such capacitive sensors cannot be mechanically protected because physical contact on the surface of the integrated circuit with the finger is necessary. However, some scratch resistance protection for the capacitive sensor electrodes is required to prevent xe2x80x9cscratchxe2x80x9d damage to the sensor electrodes. Such damage typically results from undue (and unnecessary) pressure on the surface of the integrated circuit, alone or in combination with some sharp edge or protrusion such as a callous or scar, fingernail, dust or dirt particle, etc. While extremely hard passivation stacks employing silicon carbide (SiC) have been developed for these circuits, the capacitive electrodes may still become very badly damaged through use.
It would be desirable, therefore, to provide a mechanism for preventing damage to integrated circuits resulting from required contact with a sensor portion of the integrated circuit.
In capacitive sensor circuits where physical contact is required and excess pressure may be inadvertently applied to the sensor surface, aluminum is not sufficiently hard to provide xe2x80x9cscratchxe2x80x9d protection and may delaminate, causing circuit failure even if passivation integrity remains intact. Because hard passivation layers alone provide insufficient scratch resistance, all metallization levels within the sensor circuit between the surface and the active regions of the substrate are formed of a conductive material having a hardness greater than that of aluminum. The selected conductive material preferably has a hardness which is at least as great as the lowest hardness for any interlevel dielectric or passivation material employed. The selected conductive material is employed for each metallization level between the surface and the active regions, including contacts and vias, landing pads, interconnects, capacitive electrodes, and electrostatic discharge protection lines. Tungsten is a suitable conductive material, for which existing processes may be substituted in place of aluminum metallization processes.